Passing type-parameterized function to ArgumentError.checkNotNull broken in 2.8.1

[jonasfj]

I might be using more type-parameters than is useful :)
But I ran into this curious case, and wonder if it's a known regression.

typedef WrapperFn = Future<T> Function<T>(Future<T> Function() fn);

void acceptsWrapper(WrapperFn wrap) async {
  // This works on Dart 2.7.0, but not Dart 2.8.1
  ArgumentError.checkNotNull(wrap);

  // This works fine on both Dart 2.7.0 and 2.8.1
  if (wrap == null) {
    throw ArgumentError.notNull();
  }

  await wrap(() async {
    // Do something async that we want [wrap] to be able to initiate and
    // return the result from, including catching exceptions.
  });
}

void main() => print('hello world');

On dart 2.7.0 it works, but on 2.8.1 I get (before it starts running):

hack.dart:5:17: Error: Generic function type 'Future<T> Function<T>(Future<T> Function())' inferred as a type argument.
 - 'Future' is from 'dart:async'.
Try providing a non-generic function type explicitly.
  ArgumentError.checkNotNull(wrap);
)

[jonasfj]

@lrhn might this have to do with signature change from dart 2.7.1 which has:

static void checkNotNull(Object argument, [String name]);

to dart 2.8.1 which has:

static T checkNotNull<@Since("2.8") T>(T argument, [String name]);

[lrhn]

It is because of that change, yes. I did not anticipate that change being breaking. Just goes to prove that no change is too small to break someone!

Dart prohibits a generic function type from being a type argument.
However, it does not stop itself from inferring such a type as a type argument.

We should just stop having that restriction, or at least stop inferring generic types as arguments (just inferring Function would work).

The workaround would be to pass an explicit type argument (just void would be fine, since no-one is using the return value).

[jonasfj]

For work around, one can also do:

if (wrap == null) {
  throw ArgumentError.notNull();
}

That works fine :)

But for existing code fixing this would be nice, at it's a static error for any code using package:acyclic_steps (I'll probably publish a patch version with the work around).

[lrhn]

Closing in favor of dart-lang/language#496

[mit-mit]

This is supported as of Dart 2.14.